Improve docs for SCHEDULE_EXACT_ALARM

Mentioning protection level and explicitly stating that the permission
is revocable.
Updated the hyperlink to a page with more comprehensive guidance.

Test: m offline-sdk-docs && \
  cd $ANDROID_BUILD_TOP/out/target/common/docs/offline-sdk/ && \
  python3 -m http.server 8000
Then manually inspect the contents of
http://localhost:8000/reference/android/Manifest.permission.html#SCHEDULE_EXACT_ALARM

Fixes: 205210647
Change-Id: Ide4c42ed22ee63df90c466f89331e5680cc39660